This study is a continuation of work designed to assess cytokine gene expression using RT-PCR analysis in the nonhuman primate (rhesus) experimentally infected with simian malaria species Plasmodium cynomolgi (n=3) and P. knowlesi (n=3). PBMC were isolated and total RNA extracted from monkeys at baseline, 3 times at 4-day intervals during the acute infection with the respective Plasmodium species, and once during the convalescent phase 1 week post-treatment. The monkeys were then re-infected to assess whether immune responses stimulated during the early, initial infection could enhance parasite clearance. PBMC were isolated and RNA was prepared as above. Parasitemias were monitored daily and plasma collected simultaneously with PBMC for determination of IFA titers. Monkeys infected with P. cynomolgi showed similar cytokine expression profiles strong IL-12 expression was seen after infection, continuing through parasitemic episodes then returning to baseline levels during post-treatment convalescence periods; IL-10 expression was prominent and paralleled that of IL-12; 2 of 3 monkeys showed moderately elevated IFN- and TNF-` during ascending parasitemias; biphasic expression of IL-4 was observed after each infection with the peak level of IL-4 mRNA transcripts associated with peak antibody titers. A different cytokine expression pattern was seen in monkeys infected with P. knowlesi, a rapid, fulminating malaria. Primary infection was characterized by early IL-12 expression followed by IFN- which predominated at peak and descending parasitemias. TNF-` was moderately expressed during both infection periods at ascending parasitemias; expression was inversely proportional to parasitemia levels and IFA titers. IL-10 expression was moderate and correlated with that of IFN-during primary infection. IL-4 transcripts were detected only after secondary infection (unlike the biphasic expression observed in P. cynomolgi-infected monkeys), coincident with parasite clearance and maximal IFA titers. Future studies will target the association of cytokine production with specific antimalarial